1. Field of the Invention
The present invention relates to an engine control system for a jet-propulsion boat, to a jet-propulsion boat incorporating the engine control system, and to a method of using the engine control system. More particularly, the present invention relates to an engine control system for a jet-propulsion boat, which is capable of preventing cavitation regardless of whether the engine of the jet-propulsion boat is provided with a forced-induction system.
2. Description of the Background Art
A boat which is commonly known as a “jet-propulsion boat” is propelled by a reaction force produced when a jet of water that has been pressurized and accelerated by a water-jet pump is ejected from a jet-out port provided at a rear portion of the water-jet pump. A jet-propulsion boat causes cavitation due to its own characteristics. Specifically, in a case where the jet-propulsion boat sails slowly or tows a load, a rapid acceleration of an engine by opening a throttle valve of the engine at a large opening results in a situation where the speed of the vessel cannot catch up with the rapid acceleration of the engine.
In this situation, the jet pump cannot take a sufficient amount of water, and thus the pump load becomes excessively small, which results in an occurrence of a cavitation. With the an occurrence of a cavitation, the engine accelerates nearly up to the upper-limit engine speed, which in turn activates a limiter in order to decelerate the engine speed.
When the engine speed slows down to a target speed or slower, the engine starts to accelerate again. As the acceleration and the deceleration of the engine speed are repeated, the speed of the vessel is raised up to a level that is fast enough to make equilibrium between the amount of water taken into the pump and the output of the engine. Such equilibrium makes the engine speed stable. Due to an occurrence of cavitation, engine-speed hunting occurs until the engine speed becomes stabilized, and thereby the acceleration of the jet-propulsion boat is hindered at its startup.
For dealing with such drawbacks, e.g., an occurrence of a cavitation, the Japanese Patent Application Publication No. 2004-324483 discloses an engine control system for a jet-propulsion boat. When the acceleration of the engine speed reaches at a predetermined rate or higher, the engine control system according to the Japanese Patent Application Publication No. 2004-324483, retards rising of a boost pressure of a turbocharger (forced-induction system), and thereby holds down the acceleration of the engine speed.
In addition, the Japanese Patent Application Publication No. Hei 10-141105 discloses a traction-control system which controls a skid of a driving wheel by reducing a driving power of an engine when the driving wheel skids and when the vehicle is running. The traction control system includes a sensor for detecting a rotary speed of a gear in a transmission unit. The traction control system also includes ignition-timing control means for retarding an ignition timing of the engine when a rate of rising of the rotary speed of the gear detected by the sensor exceeds a predetermined rate.
The engine control system for a jet-propulsion boat disclosed in the Japanese Patent Application Publication No. 2004-324483 is a system to control the boost pressure of the forced-induction system, and therefore, is not applicable to an engine with no forced-induction system.
The traction control system as disclosed in the Japanese Patent Application Publication No. Hei 10-141105, is designed to control the engine speed, and to reduce the output of the engine based on the rising rate of the rotary speed of the gear. Accordingly, the traction control system thus configured is not suitable for controlling an engine of a jet-propulsion boat in which an impeller of the boat is directly connected to an output shaft of the engine. In addition, the system disclosed in the Japanese Patent Application Publication No. Hei 10-141105 is not supposed to be used for the purpose of preventing the cavitation that is likely to occur when the boat is starting.
The present invention has been made to overcome such drawbacks of the existing engine control system for a jet propulsion engine. Accordingly, it is one of the objects of the present invention to provide an engine control system for a jet-propulsion boat, which is capable of preventing an occurrence of cavitation regardless of whether the engine of the jet-propulsion boat is provided with a forced-induction system.